Known laser marking methods are disclosed, for example, in PTL 1 and PTL 2.
PTL 1 discloses forming a clear and hard-to-discolor marked part by laser marking in such a manner that a reaction coloring layer containing a colorant reactive with a laser beam is formed on the surface of a dyeable plastic substrate, a dyeing layer is formed with a combination color that makes the ground color of the coloring layer deeper by dyeing, and marking is performed by irradiation with a laser beam.
PTL 2 proposes a laser marking method in which the surface of a resin molded body is irradiated with a high-energy pulsed laser to mark a pattern, letter, or symbol of any form, wherein a resin powder containing inorganic particles is applied to the surface of the resin molded body, and a laser beam with a pulse of 1 kHz to 50 kHz is applied in conformity with a marking shape, thereby clearly developing the marking shape on the surface of the resin molded body.
However, the techniques described in PTL 1 and PTL 2 have a fatal defect in that marks formed by the laser marking methods are significantly inferior in clarity and solidity.
PTL 3 proposes a technique that overcomes this defect. According to PTL 3, a coloring-material-containing layer in which the coloring material is dispersed is formed on the surface of a substrate made of thermoplastic material, and a laser beam is applied to the coloring-material-containing layer in conformity with a predetermined marking shape to soften the portion of the substrate irradiated with the laser beam, so that the coloring material of the coloring-material-containing layer is mixed in the softened portion, thereby developing the predetermined marking shape on the surface of the substrate.
In this method, however, the formed coloring-material-containing layer is thick, and therefore, high output energy is required to fix the coloring material in the surface of the substrate by irradiation with a laser beam. As the output energy of a laser beam increases, the laser fluence also increases; consequently, it is inevitable that the size of the resulting mark becomes larger. Accordingly, it was difficult, even by the method of PTL 3, to draw very minute letters or patterns (e.g., about 20 μm in width) on substrate surfaces.